Accurate First-Principles Electrochemical Phase Diagrams for Ti Oxides from Density Functional Calculations

TL;DR

This paper presents a workflow for accurately calculating electrochemical phase diagrams of Ti oxides using density functional theory, emphasizing the importance of exchange-correlation functionals and thermal effects for reliable predictions.

Contribution

It introduces a systematic method combining advanced functionals and vibrational contributions to improve the accuracy of first-principles electrochemical phase diagrams.

Findings

MetaGGA and hybrid functionals improve diagram accuracy.

Thermal vibrational effects are essential for realistic diagrams.

Using experimental formation energies at room temperature can cause inaccuracies.

Abstract

Developing an accurate simulation method for the electrochemical stability of solids, as well as understanding the physics related with its accuracy, is critically important for improving the performance of compounds and predicting the stability of new materials in aqueous environments. Herein we propose a workflow for the accurate calculation of first-principles electrochemical phase (Pourbaix) diagrams. With this scheme, we study the electrochemical stabilities of Ti and Ti oxides using density-functional theory. First, we find the accuracy of an exchange-correlation functional in predicting formation energies and electrochemical stabilities is closely related with the electronic exchange interaction therein. Second, the metaGGA and hybrid functionals with a more precise description of the electronic exchange interaction lead to a systematic improvement in the accuracy of the Pourbaix diagrams. Furthermore, we show that accurate Ti Pourbaix diagrams also require that thermal effects are included through vibrational contributions to the free energy. We then use these diagrams to explain various experimental electrochemical phenomena for the Ti--O system, and show that if experimental formation energies for Ti oxides, which contain contributions from defects owing to their generation at high (combustion) temperatures, are directly used to predict room temperature Pourbaix diagrams then significant inaccuracies result. In contrast, the formation energies from accurate first-principles calculations, e.g., using metaGGA and hybrid functionals, are found to be more reliable. Finally to facilitate the future application of our accurate electrochemical phase equilibria diagrams, the variation of the Ti Pourbaix diagrams with aqueous ion concentration is also provided.